1) Rush to a lot take practice tests before learning anything. - Waste of Tests.
You should take a maximum of 1 test just to understand where you stand in terms of your current preparation.
Better take up OG diagnotsic section at the start of the book.
2) Begining to practice with Official Guide - Waste of OG GMAT questions.
Try to learn the basics from the supplement books and pratice supplement book questions.
and gradually move to OG questions.
3) Giving GMAT practice or actual exam at the worst time during the day such as studying after a long day. Or Taking the test in the evening slot when you are accustomed to Sleep During those hours.
4) Not Making An Error Log. You should always make an error log, because you are likely to forget grammar mistakes. If you have an error log, you will have an index of these mistakes and you can avoid them in the Exam. [ Read More: How to Make a GMAT Error Log]
5) Making Unrealistic GMAT Score Targets. It is good to have a 700+ score but it is not the only thing, what matters with 700-720 score is your overall profile. And moreover only a fraction of students score above 760 on the GMAT.
6) Starting to prepare with Poor English Proficiency.